Bone screws are used, for example, for osteosynthesis either in conjunction with bone plates or also individually. Small fragment screws, in particular, also so-called cortex screws, are used after cranial injuries following accidents as well as in orthopedic maxillofacial surgery, in hand surgery and, in various dimensions, in other areas as well. Such small fragment screws are extremely difficult to handle because of their small dimensions. The length of such small fragment screws is approximately in the range of 8 mm to 10 mm, and the screw head may have a maximum diameter of about 3-4 mm and the screw shank may have a diameter of 1.5-2.5 mm. It is easy to imagine that such small fragment screws can be handled by means of a screwdriver with extreme difficulty only, because gripping by hand and attaching the screwdriver at the same time and the screwing in of such small fragment screws is difficult.
Various systems, by which the handling of such small fragment screws is said to be simplified, have now become known for making it possible to handle such small fragment screws in a simple manner at the site of use. Systems that are characterized, due to a special embodiment of the screwdriver, by the fact that the small fragment screws can be grasped by means of gripping elements at the screw head such that the small fragment screw or bone screw cannot slip off from the tool blades of the screwdriver any longer, have become known, for example, from U.S. Pat. No. 5,649,931, DE 38 04 749 A1, DE 100 44 714 C2, and DE 35 39 502 C1.
For example, a screwdriver is provided in the subject of DE 35 39 502 C1, which is provided with a tool, which is seated at a shank and has a plurality of tool blades, with which the tool can be caused to engage corresponding blade slots of the screw head of the bone screw in a positive-locking manner. Furthermore, a screw removal means, which has a gripping member, by means of which a screw head can be grasped, is provided at the shank. This gripping member is actuated by means of a clamping sleeve displaceable along the shank, wherein the gripping members elastically project from the shank radially in the outward direction when the clamping sleeve is retracted. When the clamping sleeve is displaced in the direction of the tool seated at the end of the shank, the gripping members are pressed by the clamping sleeve radially inwardly, so that these essentially hook-shaped gripping members extend behind the screw head and the latter is held snugly against the tool at the end of the shank. The subjects of the other documents mentioned above have a similar embodiment as well.
It has now been found that the handling of such special screwdrivers is to complicated insofar as these screwdrivers must first be placed with the tool blades of their tool on the screw head and the clamping sleeve must subsequently be displaced along the shank in order to grasp the screw head of the bone screw. Since the hook-shaped gripping members extend behind the screw head, it is not possible with this tool to screw in a bone screw completely. Shortly before the maximum screwing-in depth is reached, the clamping sleeve must be retracted again in order to release the screw head. The gripping members also must be subsequently retracted axially, so that they are not in contact with the bone when the bone screw is screwed in further and the further, complete screwing in of the bone screw is not hindered. At the same time, the gripping members must be withdrawn from their position in which they project axially over the tool in order not to hinder the complete screwing in. However, if the clamping sleeve is brought into an inactive position together with the gripping members, the screwdriver with its blade-like tool can slip off, as a result of which the safety of operation is diminished.
DE 296 11 140 U1 discloses a system comprising a screwdriver and a bone screw, in which the tool of the screwdriver has a central hexagon, which is arranged between two radially outwardly directed tool blades. This hexagon minimally projects over the tool blades in the axial direction and can be caused to engage a corresponding hexagon socket of the bone screw. The hexagon of the tool has a conical shape, so that this hexagon is clampingly accommodated in the hexagon socket of the bone screw when it is correspondingly pressed against the bone screw. The drawback of this embodiment is that after the hexagon of the tool has been inserted several times into a hexagon socket of the bone screw, a burr is formed at the hexagon of the tool, so that this tool is no longer able to function after several uses, because it can no longer be inserted into the hexagon socket of the bone screws deep enough, but is in contact with the burr. However, a clamping hold also cannot thus be achieved any longer.
Clamping elements, which are mounted elastically as separate components at the tool, are used in the subject of DE 93 10 668.8. On the one hand, a clamping ball is provided here, which is inserted into a hole of the tool. A hexagon socket or even a hexagon insert bit may be provided here as the tool. It is disadvantageous here that this embodiment cannot be used in case of extremely small bone screws, because the clamping elements must always have an extremely small axial distance from the outer front side of the tool. The consequence of this is that the bone screws would have to have a correspondingly long hexagon insert bit or hexagon socket to enable the clamping element to clampingly engage the bone screw at all. However, a great axial length of the hexagon insert bit or hexagon socket is not possible in case of extremely small bone screws, because the dimensions of these screws must be extremely small for use in microsurgery.
DE 91 10 576 U1 describes a screwdriver, in which a spring bar, which is to be clampingly engaged with the blade slots of the bone screw by its radial spring force, is provided within the tool shank. The handling of this design is also extremely uncertain, because reliable clamping cannot always be achieved. In addition, this design cannot be embodied in the case of the aforementioned extremely small bone screws, because the tool shank, with its extremely small diameter, does not permit the arrangement of such a spring bar.
The subject of FR 2 723 839 is likewise unsuitable for small bone screws. A clamping cylinder projecting from the tool on the front side is provided here. This clamping cylinder has a cross slot, which is joined by outwardly radially projecting tool blades. The clamping cylinder is pressed here into an axial hole of the tool. In case of tool shanks of a small diameter, this is likewise not possible, because the stability of the tool would be compromised.